How do they make the special effects on TV like when a shop blows up just as
someone is leaving it?

Anthony Lealand, a
pyo-technic scientist who formed The Fireworks Display Company of Christchurch
and who regularly devises special effects for orchestral productions, celebrity
openings, advertisements, movies and TV programmes, responded.

Real explosions are actually quite boring looking things.

There is a fast flash, objects fly about so fast you wouldn't see them on video
and generally there is only a small puff of black smoke. This is because an
explosion is the result of a very fast chemical reaction and most explosives
are carbon rich. In other words there is not enough oxygen to burn the carbon
in them.

However this is too boring for television and film. For these explosions we
use low explosives, such as gunpowder and nitro-cellulose, which are very slow
and which therefore do not generate a supersonic bang. These are triggered by
electrical ignition so the sequence of explosions is under the careful control
of the explosive expert and/or his/her computer. For example, as an actor runs
from a building a whole sequence of explosions can be initiated.

For effect, these explosives are placed inside bags or drums of fuel which
ignite to give the glowing fireball effect. Frequently other pyrotechnics
effects are added, such as mines or mortars or cannons that will blow out white
stars or titanium powder to form white sparks. The procedure is messy, dangerous
and frightening to rig because the liquid fuels could be ignited by chance
sources of ignition. This can go horribly wrong unless one has had considerable
experience.

The whole effect is a comic book explosion totally unlike a real explosion but
fully satisfying the visual demands of television and film.